Gas holder water treatment



Dec. 20, 1938. J. R. SKEEN 2,141,049

GAS HOLDER WATER TREATMENT Filed Oct. 14,v 1937 2 Sheets-Sheet l v'r T AIf M. h i 1J' Lax/m cw @1123 INVENTOR ATTORN EY Dec. 20, 1938. R, SKEEN2,141,049

GAS HOLDER WATER TREATMENT Filed Oct. 14, 1937 2 SheetsSheet 2 INVENTORATTORNEY Patented Dec. 20, 1938 GAS HOLDER. WATER TREATMENT John R.Skeen, Philadelphia, Pa., assignor to The United Gas ImprovementCompany, a corporation of Pennsylvania Application October 14, 1937,Serial No. 168,911

14 Claims.

This invention pertains generally to the control of bacterial growth andpertains particularly to the reduction or prevention of discoloration ofsurfaces and corrosion of iron in iron-water systems.

The invention will be described in connection with gas holders used inthe gas industry. However, it is to be understood that it may have manyother applications.

For many years the gas industry has been concerned with the appearanceof gas holders and has sought to make such holders as inoffensive inappearance as possible.

As a result the'exposed surfaces are painted, partly for protection, butlargely for appearance.

This presents no particular problem in the case of pressure andwaterless holders since the maintenance of the paint is no moredifficult than the maintenance of the paint on any other structuralsurface.

In the case of water sealed holders, however, an entirely differentproblem arises. Surfaces which are intermittently immersed in water withthe rise and fall of the lift or lifts acquire a reddish browndiscoloration, very unsightly in appearance. In warm weather it mayrequire only a few weeks for a freely painted surface to be almostcompletely masked with a reddish brown film.

The industry has heretofore merely resorted to palliatives such asmatching the paint to the discoloration, spreading oil on the watersurface, mechanically skimming the water surface, or illtering the waterwithout arriving at the cause of the trouble.

The rusting of iron in water or moist air is of universal occurrence. Ifthe water in contact with iron is stagnant, the surface of the waterbecomes covered with a heavy, tenacious rustyappearing scum. Althoughcircumstances make a water sealed gas holder a particularly suitableplace to study the phenomenon, it occurs in the same way in a bucket ofwater or in any other similar environment.

To illustrate the problem as applied to gas holders, reference will bemade to the drawings in which:

Figure l is a sectional elevation diagrammatically illustrating a singlelift gas holder; and

Figure 2 is a sectional elevation diagrammatieally illustrating amultiple lift gas holder.

Referring now to Figure l, a simple form of water sealed gas holder isshown as consisting of an inverted bell l0 extending downward into atank H containing water l2 so that the inverted u holder is used.

bell III is free to rise and fall according to the amount of gasconfined in it.

A guide frame for the bell II) as it ascends and descends is illustratedat l3, and pipes for the introduction and withdrawal of gas above thelevel of the water l2 are illustrated at M and I5 respectively.

To increase the capacity of the holder without increasing the size ofthe tank, the multiple lift This is illustrated in Figure 2, wherein abell 20 and lifts 2!, 22 and 23 are shown in an expansible-collapsiblearrangement which extends down into tank 26. A guide frame isillustrated at 25 and pipes to introduce and to draw off gas above thelevel of the water 26 are illustrated at 21 and 28 respectively.

The bell 20 and lifts 2i and 22 have attached to their lower peripheral,edges outwardly extending annular cups 29, 3|] and 3| respectively, andlifts 2|, 22 and 23 have attached to their upper peripheral edgesinwardly and downwardly extending grips 32, 33 and 34 respectively.

As illustrated in Figure 2, bell 20 and lift 2| are full of gas. A sealis formed between bell 20 and lift 2| as the result of the dipping of.grip 32 into water 35 in cup 29.

If more gas is now forced into the holder, the lift 2| will be raisedentirely out of the water. In rising, cup 30 containing water 36 dippedfrom tank 24 rises underneath grip 33 with grip 33 entering the cup 30to form a seal with the water 36 therein.

As more and more gas is forced into the holder each succeeding lift 22and 23, and as many others as may be provided, rises in turn until theholder istfull.

'As gas is withdrawn from the holder the lifts and the bell descend inthe inverse order in which they rose. 40

It will be noted that the water level in tank 24 is such that the cupsand grips engage and disengage below the water level.

The bell, lifts and framework are usually constructed of steel. Whilesteel tanks are in general use and are usual in newer constructions,tanks have also been constructed of concrete and of brick.

In cold climates it is necessary during the winter to heat the waterinthe tank and cups to prevent it from freezing. This is usually accom--plished by introducing steam, or less often hot water, into the cups andinto the top of the tank.

The holders so far described are conventional in character.

It will be obvious that a gas holder may have any desired capacityranging from the small gasometers used in laboratory work up to thelarge holders used for supplying metropolitan centers. Holders up to twohundred and fifty feet in height have been built with tanks up to threehundred feet in diameter. The capacity may be up to twenty million cubicfeet of gas or more. The water contained in the tank may be as high astwenty million gallons or more. i I

It is not customary to paint the insides of the bell and the lifts andconsequently large surfaces of unpainted iron come into contact with anenormous volume of water and particularly that within the boundaries ofthe lower lift.

Since one or more lifts are always at least partly down except duringthe short intervals when and if the holder is completely filled, and

the lower lift is completely down during a large part of the time, forthe purposes of convenience in description the waterwithin the submergedlifts, that is, the water directly underneath the gas, will be referredto as the inside water and the water in the annular space between thesubmerged lifts and the tank, that is, the'water exposed to theatmosphere, will be referred to as the outside water".

Incidentally it should be noted that the latter is relatively small involume compared to the former, isof the same depth, and has a smallannular surface area exposed to the atmosphere compared to the largecircular surface area of the inside water which is exposed to the gas.

The inside water, beingin equilibrium at its surface with the gas, isfor the most part almost devoid of oxygen, even though oxygen issubstantially soluble in water, because of the 'very small oxygencontent of the gas. On the other hand the inside water carries arelatively high carbon dioxide content because of the relatively highcarbon dioxide content of the gas and the substantial solubility ofcarbon dioxide in water.

Contrasted to this since the outside water is in equilibrium at itssurface with air it carries a relatively high oxygen content andarelativelylow carbon dioxide content. This is, of course, becauseairhas a relatively high oxygen content and a relatively low carbon dioxidecontent. I

At intermediate points between the surface of the inside water and thesurface of the outside water, the oxygen content and the carbon dioxidecontent will be intermediate the values at the two surfaces.

This is, of course, assuming that there are no disturbing conditionssuch as rainfall, convec-,

tion or other currents, etc. However, on the whole and for the purposesof this description such conditions may be assumed to exist at least forpracticable purposes.

Although iron is relatively insoluble in water,

actually minute quantities of iron go into solution in water. Due to theabsence of oxygen and the presence of carbon dioifide in the insidewater this iron takes the form of a ferrous carbpnate namely Fe (H003):or Fe'COa, according to the following equations,

tration of ferrous carbonates is reduced in some way.

As stated above, currents of various kinds cause a certain degree ofmixing of the inside and outside water. It will be appreciated, forinstance, that the raising and lowering of the lifts producesconsiderable churning calculated to cause mixing. This is particularlytrue of the lower lift which upon being raised also reduces the boundarybetween the inside water and the outside water.

Furthermore, the laws of diffusion apply to this system, tending tobring the inside water and the outside water to the same "composition.

\ As a result of the foregoing, inside water containing ferrouscarbonates in solution becomes mixed in the annular space outside thelowered lifts with the outside water which carries oxygen in solution,and one or more of the following reactions take place.

. It will be noted that ferric hydroxide is a product of each of thesereactions.

Ferric hydroxide is characterized by its insolubility in water, by itsyellowish brown color, and by its flocculent or colloidal nature.

While'ferric hydroxide may be dehydrated to the oxide according to thefollowing equation 2Fe(OH)3=FezO:+3Hz0, this is not thought to takeplace to a great extent due to the large volume of water present whichwould tend to drive the latter reaction to the left. At most, hydratedoxides F82O3(H2O)n are formed. However, all of these compounds'are veryinsoluble,

are deeply colored and may take part in the distion of the inside wateris not appreciably effected,

first, because the total volume of inside water is so many'times greaterthan that of the outside water, and second, because a large part of suchoxygen is lost to the vapor space above the water in establishingequilibrium conditions.

Such outside water, however, serves to unsaturate the inside water inferrous carbonates by dilution and consequently more iron is dissolved.

If the highly colored ferric hydroxide, ferric oxide and/or hydratedferric oxide would settle out of the outside water at a rapid rate,there would be no problem of' holder discoloration.

Unfortunately the cycle is complicated by the presence of amicro-organism belonging to the plant family commonly designated as ironbacteria. ,The group has the peculiar habit of thriving in the presenceof iron.

Green plants obtain their energy for growth from sunshine and almost allother living things obtain their energy for growth from green plants.

One of the few possible exceptions is the group iron bacteria, themembers of which may obtain their energy for growth from the oxidationof ferrous iron to ferric iron, although it appears that they cannotthrive unless oxygen is also present.

Although iron bacteria are well known, it was not known prior to myinvention that they were concerned either with holder discoloration or,as far as I am aware, with the ordinary processes of iron rusting.

I have reason to believe that the particular micro-organism involved,which I prefer to call ugiensis, differs from the forms of iron bacteriapreviously identified.

That an organism associated with the well known phenomenon of ironrusting has apparently remained unidentified for so long a time can beexplained by a combination of facts. First,

face of the outside water where oxygen is in solution in relativelylarge quantities, its growth decreasing with depth because of decreasingconcentration of oxygen. Colonies adhere to unpainted metal surfacessuchas the inside surfaces of the tank and of the submerged lifts (butperhaps not of the bell) below the water line.

The corrosion of iron is greatly accelerated: by the absorption offerrous carbonates from solution and their deposition within the plantbody as hydrated ferric oxides, and due to stimulation of irondissolution from those surfaces immediately beneath the colonies by theacidic reaction resulting from their normal metabolism. I v v Thesolution ferrous iron collects in the gelatinous sheaths around thecells of the organism and when oxidized to the ferric form produces aniron colloid with the dead cell materials.

It is this iron-organic complex that gives rise to the continuousgelatinous scums on the surface of the outside water.

As a lift emerges from the water during the inflation of the holder thelayer of scum sticks to the paint. I

This scum is extremely difiicult to remove while still wet and uponbecoming dry cannot be removed without destroying the paint film.

As a matter of fact the scum may be likened to a paint in which the ironrust constitutes the pigment, the organic debris the vehicle, and thewater the thinner.

Each time a lift of the holder is immersed and then raised this skimmingprocess is repeated until the paint underneath is eventually completelymasked.

A secondary phenomenon of this organism also contributes to thedisfiguration of the holder paint: The peripheral cells of a colonyadhering to an unpainted metal surface become brittle with accretions offerric iron. Agitation causes dislodgment and a yellow sediment thusformed diffuses through the outside water-toward the liftsasit settlesto the bottom of the tank, and some of it becomes lodged on rivet headsand other projections on the lifts.

This sediment, upon drying, adheres almost as tenaciously as the driedscum.

If it were not for the iron bacteria the insoluble ferric iron compoundswould settle to the bottom of the tank without causing any seriousdiscoloration.

Having ascertained the basic cause of the dimculty, I have solved it bycreating an unsuitable environment for the growth of the organism ororganisms responsible for .the discoloration.

I find that increasing the pH of the water, for instance by the additionof sodium bicarbonate, reduces but does not prevent scum formation.Furthermore, even such weakly alkaline solutions attack many paint filmsso severely as to desroy them quickly.

I find further that the addition of common bactericidal reagents such asphenol, arsenic, copper, mercury salts, etc., along with an increase inthe pH of the water does not affect an improvement over the above sincethe variety of iron bacteria involved is found to be extraordinarilyresistant to the common chemicals that are toxic to most forms of life.Various types of the newer fungicides and bactericides proved equallyineffective.

Conditions most favorable to these or anic growths are warmth and aneutral or slightly alkaline water. The optimum temperature appears tobe about 85 F. which explains why more trouble has been experiencedduring the summer months.

The propagation rate decreases rapidly as the solution becomes morealkaline or more acid.

For instance, should the water be maintained with a pH of 4.5 or less,the growth of the organsm would not be troublesome, but unfortunatelythe rate of iron corrosion is greatly increased.

Similarly should the water be maintained with a pI-l of 8.0 or more, therate of growth would be reduced (but not suppressed), but unfortunatelythis is not only severe on paint films but is also somewhat diflicultand expensive to maintain since the absorption by the water of carbondioxide tends to prevent the maintenance of a high pH.

Upon investigating the growth of the organism in solutions of varyingpI-l, I have discovered that only very small concentrations of chromateion obtained for instance by the addition of sodium dchromate arerequired in solutions of above '7 and say between 7.4 and 7.9 pH, tocontrol satisfactorily the growth of the organism even at the optimumtemperature of 85 F.

I find that a very satisfactory way to maintain a desired pH in thepresence of chromate ion is by the addition of the required amount of asuitable substance such as sodium bicarbonate or sodium carbonate.

I find further that within limits the concentration of either componentmay be reduced if the amount of the other component present isincreased. In this case I prefer that the product of the two added saltconcentrations exceed a given minimum value and find that this will keeporganic growthbelow the scum forming level.

Curiously enough the chromate ion reaches its opt-mum effectiveness at apH which in the absence of 'chromate ion is distinctly favorable togrowth of the organism.

On the other hand if sodium dichromate were used alone it would not onlybe ineffective but would practically disappear'from the water in a fewweeks.

In combination the two ions (chromate and hydroxyl) show aneffectiveness which is unequalled by any other known treatment.

When the dichromate is first added to water '(which usually contains atleast some oxidizable components in solution or suspension) there isvary these figures which are given merely for theonly a small loss dueto reduction after which very little further loss is experienced.

Since treatment of such large volumes of water requires comparativelylarge quantities of reagents, cost becomes a consideration.

From this standpoint I wish to point 'out that a small amount of growthis permissible without causing noticeable discoloration and that thewater need not be so toxic to the organism as to prohibit completely itsgrowth.

Furthermore, the presence of some chromate ion under such circumstancesthat the pH is greater than 7, and particularly when greater thanapproximately 7.4, will have a measure of beneficial action in that itwill reduce to some extent at least the rate of growth of the organismeven though it may not be sufllcient to prevent the formation of scumaltogether.

Furthermore, I find that with increase in holder size smaller andsmaller concentrations of the reagents are su'fllcient to preventdiscoloration. I believe this to be due to a decreasing ratio of totalarea of unpainted surface to the total volume of water present. In otherwords, as holders increase in diameter, the area of the inside surfacesof the lifts and unpainted surfacw of the tank do not increase inproportion to the volume of water in the tank. Moreover, the width oftheannulus between the tank wall and the lifts increases with increase insize of the holder.

As an example, for instance, in the case .of large holders, that isholders of a gas capacity considerably greater than two million cubicfeet,

I find that growth on unpainted metal surfaces may be permissible up toabout one per cent of the area of such surfaces on which growth takesplace, whereas in the case of holders of a gas capacity of considerablyless than two million cubic feet such growth-should possibly berestricted to about 0.15 per cent to obtain absolute assurance againstany possible discoloration.

It is understood, of course, that conditions may centraticns-therelative amounts of each can be varied over a rather wide range.

This is illustrated in the following table in which minimum recommendedconcentrations are given, not only individually but also in terms oftheir product.

line show ranges from the smallestto the largest .holders of theparticular holder size bracket, and are based on a permissible ironcorrosion of 16% of normal for the smallest holder to 20% of normal forthe largest holder of the bracket;

Similarly the values for the third horizontal line were obtained, thepermissible iron corrosion being less than 16% of normal.

It will be noted that the product of the minimum values given for sodiumbicarbonate and sodium dichromate does not equal the recomprinciples forobtaining complete protection against discoloration with the minimumamount of chemical additions.

I prefer to initially add chromate ion in the I form of sodiumdichromate. suitablechromium compoundmay be employed.

As chromate ion slowly disappears (by reduction and overflow)- its.concentration, may be maintained, if desired, by the addition of chromicacid, by the addition of more sodium dichroinate,.

or by the addition of any other suitable substance. Chromic acid mightbe used in the first instance, except that at present it is considerablymore expensive. v

I prefer to maintain the desired pH of the water by the additionofsodium bicarbonate, or a sodium bicarbonate yielding substance such assodium carbonate. I find for instance that cost may be reduced by addingsodium carbonate, which is less expensive.

The relatively high concentration of ,carbon dioxide in the insidewater, which as above explained is maintained by the presence of carbondioxide in the confined gas, will gradually convert the sodium carbonateinto sodium bicarbonate.

However, since sodium carbonate will produce a higher pH which will bemore severe on ordinary paints than the pH produced by sodium bi- TABLEBatlio water R800 d dmini R ded ni vo ume mmen e eeommen ml mum Gascapacitiy (If holder in galloillstlo1 fi g gsi ggg mfg mumlgglllfigiligot godgufitgi gTHalgJ O; and;

on o co un colleen on percen a; i 2 pereen sm soes in in percent byweight by weight concentrations square feet 2,000,000 d up 300 320 x 10-f 24 x 10- 2.28 x 10- 2,000,000 ownto300,000 311M025 320xl0- to400x10'24X10 t032X10' 2.28Xl0' t03fllX10'P Below 300,000 I 25 400 x l0- 32 110- 320 x 10.

In explanation of the table, the first column lists ranges of holdersizes which of necessity are arbitrarily chosen. Accordingly, the valuesgiven in thelfirst horizontal line for holders having a gas capacity of2,000,000 cubic feet and up are the minimum values for a holder of2,000,000 cubic feet gas capacity. These values are based on apermissible corrosion which is 20% of normal, and may be decreased withincrease in holder size following the principles above set forth.

Likewise, the figures in the second horizontal carbonate, with gasessuch as oil gas, natural gas or other gases containing much lowerconcentrations of carbon dioxide than coal gas, producer gas or watergas, it may be necessary to use bicarbonate initially, unless, ofcourse, a special alkali resisting paint is used on the surfaces whichare submerged and also painted.

Any other suitable substance or substances may be substituted formaintaining the desired pH of the holder water.

Attention is, however, directed to the fact that However, any otherwithdrawal.

many substances which might be otherwise suitable for the addition ofchromate ionand for the maintenance of the desired pH, have componentspresent which might tend to function as plant oods.

For instance, it is commonly known that certain phosphorus, potassium,nitrogen, calcium, iron and magnesium compounds are plant foods.

Therefore, in selecting a compound to be substituted considerationshould be given to its possible effect upon the stimulation of growth ofthe organisms which it is desired to control orv destroy.

In selecting such substitute compounds consideration must obviouslyalsobe given to the question of solubility. and compatability as well asrelative cost.

In connection with the question of compatability, consideration shouldbe given to the fact that there are present in the gases substances suchas cyanogen and hydrogen sulfide and that reactions with thesesubstances is usually to be avoided in the holder.

When the final selection is made the substances chosen may be added tothe holder water in any desired manner.

I have added carbonate and dichromateby separate solution in waterfollowed by pumping into the holder tank.

Some consideration should be given to mixing with the holder water.

I have accomplished this by drawing the mixing water in batches from thebottom of the tank and when almost saturated with the substance beingadmitted pumping it back into the tank at a point within the lifts, nearthe top, and near the opposite side of the tank from the point of Theprocess is 'repeateduntilthe prescribed mass of treating substanceshasbeen added.

Any other method of adding the treating substances may obviously besubstituted.

Thorough mixing, however, isioi' importance to the life of the chromate.v

If sodium carbonate is'used instead of the bicarbonate and if the painton the lifts is not alkali resisting, I recommend that care be taken asto the length of time that the lifts are immersed in order to avoiddestruction of the paint. The reduction in pI-I due to conversion of thesodium carbonate into sodium bicarbonate by carbon dioxide as well asthe efiiciency of mixing should be checked at suitable intervals untilat least a substantial part of the normal carbonate has been convertedinto bicarbonate.

Since practically all holder waters contain calcium and magnesium saltswhich are precipitated .by sodium carbonate, I recommend that the liftsbe fully inflated during the addition of the treating substances toprevent discoloration by deposition of the precipitated materials onpainted surfaces.

I have found that an average of about two weeks is required for completeclarification of the water from such precipitated substances.

The treated water will enter the cups upon immersion of the lifts and Irecommend that attention be given to the dilution of the water in thecups as a result of rainfall or of steaming to prevent freezing, inwinter. Circulation of tank water through the-- cups might be provided,or the lifts might be immersed, or the water in the cups might bebrought back to the desired composition by any other suitable means.

It appears that the chromate will react with lead paints unfavorably.Therefore, I recomcovered with two or more coats of lead-free paint inwhich case no trouble will usually be encountered.

There are now available many lead-free paints which may be used forprimers and top coats.

During the initiation of the treatment the organism in a desperateeffort to survive develops and grows with amazing rapidity in a greatlymodified form and appears as isolated colonies adhering to the unpaintedmetal. These colonies die, become brittle and disintegrate, causing someturbidity and sediment. The metal underneath the colonies appears etchedbut fortunately the etching process does not continue for more than afew weeks after the treatment is installed, with no appreciable harm tothe metal.

After the treatment has been installed and sodium carbonate has beenconverted to bicarbonate, it is thought that the inside water near itssurface is actually mildly acidic with a pH in the neighborhood of about6.7. This is because of the high carbon dioxide content of the water. Inthis case, chromium is probably present as dichromate.

On the other hand, the outside water near,

and for a considerable distancebelow, its surface is distinctly alkalinewith a pH determined almost 1 The absence of yellow, and the presence ofa clear blue color, in the outside water substantiates this viewpoint.

-In this connection I wish to point out that the organism is capable ofgrowing only in the presence of oxygen, that is, in theoutside water fora limited distance below its surface, say, about fifteen feet. Over thisdistance the water is not only alkaline but is of the clearest blue,indicating the presence of the chromate ion instead of the dichromateion.

At the surface of the outside water with the concentrations suggestedfor a holder of 2,000,000 cubic feet capacity the pH is in theneighborhood of approximately 7.85.

For the purpose of this specification and the claims, the term ironwhere used is intended to include not only iron itself but the variousforms of iron and steel, including alloys susceptible to corrosion bythe process of rusting.

While particular reference has been made to gas holders, it is to beunderstood that the invention is not limited thereto and may be appliedgenerally to the reduction of discoloration and/or corrosion wheresimilar problems are involved. By the term annulus as used in the claimsis meant the space in the tank in which the holder water is exposed tothe atmosphere at its uppersurface.

. It is, therefore, to be understood. that the particular description.is by way ofillustration and that changes, omissions, additions,substitutions and/or modifications may be made within the scope of theclaims without-departing from the spirit of the invention which isintended to be limited only as required by the prior art.

I claim:

1. A method for the reduction of the growth of iron bacteria and theformation of scum in an iron-water-oxygen system in the annulus of awater sealed gas holder containing gas, comprising maintaining saidwater in an alkaline state in the presence of chromate ion.

2. 'A method for the reduction of the growth of iron bacteria and theformation of scum in an iron-water-oxygen system in the annulus of awater sealed gas holder containing gas and having painted surfacesexposed to said system, comprising maintaining said water with a pH inthe neighborhood of 7.4 to 7.9, and with a concentration of chromate ionsuiiicient to be materially toxic to iron bacteria.

3. A method for reducing the growth of iron bacteria and the formationof scum in an ironwater system exposed to the atmosphere in the annulusof a water sealed gas holder having painted surfaces exposed to saidsystem, comprising adding to said water substances capable ofmaintaining the pH in at least the upper part of said annulus above -7but below 8 when said gas holder contains gas, and of producing chromateion in solution.

4. A method for reducing the growth of iron bacteria and the formationof scum in the annulus of a water sealed gas holder having paintedlifts, comprising adding to the water in the tank of said gas holdersodium bicarbonate and sodium dichromate.

5. A method for reducing the growth of iron bacteria and the formationof scu'm in the annulus of a water sealed gas holder containing gasincluding carbon dioxide and having painted lifts, comprising adding tothe water in the tank of said gas holder sodium carbonate and-sodiumdichromate, and maintaining the lifts in raised position until saidcarbon dioxide in said gas converts at least a substantial portionof-said sodium carbonate to sodium bicarbonate.

6. A method for reducing growth of iron bacteria and the formation ofscum in the annulus of a watersealed gas holder having painted lifts,comprising adding to the water in the tank of said gas holder sodiumbicarbonate and chromic acid.

7. A method for reducing growth of iron bacteria and the formation ofscum in the annulus of a water sealed 'gas holder containing gasincluding carbon dioxide and having painted lifts,

comprising adding to the water in the tank of said gas holder sodiumcarbonate and chromic acid, and maintaining the lifts in raised positionuntil said carbon dioxide'in said gas converts at least a substantialportion of said sodium carbonate to sodium bicarbonate.

8. A method for reducing discoloration of the paint on the lifts of awater sealed gas holder, comprising maintaining the water in the tank ofthe holder alkaline where exposed to the atmosphere, and adding to Saidwater one of a group consisting of chromate ion and dichromate ion.

9. A method for reducing discoloration of the paint on the lifts of awater sealed gas holder comprising maintaining that portion of the waterin the tank of the holder between the inside wall of the tank and theoutside walls of the submerged lifts and sufliciently near the surfaceto absorb any substantial quantity of oxygen with a pH above 7.0 andbelow 8.0 and with sufficient chromate ion in solution to besubstantially toxic to iron bacteria.

10. A method for reducing discoloration of the paint on the lifts of awater sealed gas holder comprising maintaining that portion of the waterin the tank of the holder between the inside wall of the tank and theoutside walls of the submerged lifts and sufliciently near the surfaceto absorb substantial amounts of oxygen with 'a pH sufficiently high toconvert dichromate ion to chromate ion but not greatly in excess of 8.0,and adding dichromate ion to the tank water.

11. A method for protecting the paint on the submerged lifts of a watersealed gas holder from discoloration comprising maintaining the water inthe annulus between the wall of the tank and the lifts and sufilcientlynear the surface to absorb substantial quantities of oxygen with a pHbelow 8 but sufficiently high to convert dichromate ion to chromate ion,and adding sufflcient dichromate ion to the tank water such that whenconverted to chromate ion in the above mentioned portion of the annulusconditions are sufficiently toxic to reduce the area of growth of ironbacteria on unpainted iron surfaces to at least 1% of the total area ofunpainted iron surfaces upon which such bacteria are capable .ofgrowing.

12. A method for reducing discoloration of paint on the lifts of watersealed gas holders of 2,000,000 cu. ft. capacity and below comprisingadding to the water of the tank hydroxyl ion at least equivalent to theaddition of 320 x 10- by weight of NaHCO3, adding to said water one of agroup consisting of chromate ion and di chromate ion at least equivalentto the addition of 24x 10-% by weight of Na2Cr-2O'z-2H2O, and increasingat least one of such additions so that the product of the'equivalentconcentrations of NaHCOa and NazCizOq-2Hz0 will be at least2.28 x 10- 13. A method for reducing discoloration of paint on the liftsof water sealed gas holders of gas holding'capacity of at least2,000,000 cu. ft. comprising adding to the water of the tank hydroxylion approximately equivalent to the addition of 320x10 by weight ofNaHCOs, adding to said water one of a group consisting of chromate ionand dichromate ion approximately equivalent to the addition of 24 x 10by weight of NazCrzOv-ZHzO, and increasing at least one of saidadditions so that the product of the equivalent concentrations of NaHCOaand NazCrzOv-2Hz0 will be at least approximately 2.28 x 10- 14. A methodfor reducing discoloration of the outer surfaces of. the lifts of watersealed gas holders, comprising maintaining that portion of the water inthe annulus which is sufllcien y near the surface to absorb anysubstantial quantity of oxygen with a pH at least approximately between7.4 and 7.9 and with sumcient chromate ion in solution'to reduce thearea of growth of iron bacteria on unpainted iron surfaces to at least1% of the total area of unpainted iron surfaces upon which such bacteriaare capable of growing.

JOHN R. SKEEN.

